The membrane attack complex (MAC) of complement is a natural effector system that can destroy invading microorganisms such as yeast and gram negative bacteria. In addition, the MAC may play an ancillary role in allograft rejection and killing of malignant cells. Complement activation results in the assembly of this cytolytic complex from soluble plasma proteins, C5 through C9. Although these proteins have been identified for a number of years structural information about them has been lacking and this has made it difficult to gain further insight into the mechanism by which complement can kill foreign cell types. The focal point of this grant proposal is the investigation of C9 structure and its involvement in membrane damage. C9 is a single chain plasma glycoprotein (Mr=65,000). When assembled as part of the MAC, C9 has been shown to traverse the bilayer, and it is the major protein associated with the hydrocarbon portion of the lipid. C9 has the remarkable capacity to polymerize spontaneously to form hollow tubules (Mr=740,000-1,000,000) which, when incorporated into membranes have the characteristic appearance of complement dependent membrane lesions. During the last year we have determined the amino acid sequence of C9, but considerable more progress is necessary if we are to understand comprehensively the structure-function relationships of this protein. The main goals of this proposal are: (1) to determine the disulfide bridging patterns; (2) to identify the regions of the protein that are responsible for association with phospholipid membranes, metal binding and self assembly; (3) to determine the location(s) and role of the carbohydrate (4) to investigate catalysis of C9 polymerization by C5b-8. The objective of this proposal is to provide a clearer understanding about the mechanism of complement lysis at a molecular level.